Air fuel (mixture) ratio display's not only look great but can be a very useful diagnostic tool for locating engine faults. They are commonly available and cost in the region of £100 (a good spend though!)

Custom Fit- 10 LED Air/Fuel Ratio Display

Mixture (or Lambda) displays work by using your cars existing ‘lambda sensor’ mounted the exhaust- these are on cars where a ‘Closed Loop Feedback’ system is incorporated to control the fuelling (see ECU article) and are fitted to most post 90 cars. For cars not fitted with lambda sensors, they are available but do cost around £70 & will require some welding to fit them in the exhaust.

How do they work?Lambda sensors produce a voltage signal that reacts to the amount of unburned oxygen in the exhaust. Essentially an oxygen sensor is a battery that generates its own voltage. When hot (above 250 degrees c.), the zirconium dioxide element in the sensor's tip produces a voltage that changes according to the amount of oxygen present in the exhaust compared to the oxygen level in the outside air. The greater the difference, the higher the sensor's output voltage which ranges from .1V to around 1V.The display itself will usually come with 3 wires: A positive (12V) usually red,A negative usually black,A signal input usually green or grey.Once you have found a suitable place to mount your display, you must connect the negative wire to an earth (Although it is possible to crimp this to an existing earth wire (e.g. clock) it is always best to use a fresh earth point on the chassis).Next get the signal wire into the engine bay through the bulkhead; best done by threading it through an existing rubber grommet etc to save drilling.Now locate your ‘lambda’ sensor. The sensor is mounted in your exhaust system, usually just after the 4 branches of the exhaust headers join. The sensors look like a metal cylinder about 20mm diameter & 40m long:

A Typical Lambda Sensor mounted in a section of exhaust.

Lambda Sensors usually have 4 wires but it isn’t un-common to have a 2, 3 or a 4 wire sensor. The sensors wires are as follows:• For a 4 wire sensor:2 wires of the same colour – this is the sensors heater (usually white wires).1 wire will be black, this is the sensors earth.1 wire will usually be grey, this is the signal wire.• 3 Wire sensor:2 white wires=heaterBlack wire=Signal.• 2 Wire sensor:Grey wire = groundBlack wire= Signal

The signal wire of your display needs to be joined to the signal wire of your lambda sensor; this is best done by cutting & soldering the wires together and suitablyinsulating the joint (if this is not possible it may be easier to use a ‘Crimp connector', these are quick and easy to use & are very reliable if done and insulated properly), remember to keep the joint as far as possible away from the exhaust to protect it from heat.Once you have finished the joint, tidy up the wires & secure them along the whole route to prevent from vibration & abrasion (especially near sharp corners & hot engine parts).Now, connect your display’s positive wire, this may be done by joining the wire to an ‘ignition’ controlled wire (e.g. cigarette lighter or direct to the ignition barrel) or you will find you battery keeps going flat! Remember it is always best to fuse the wire (3A), although the displays rarely use any more the 200milli amps, it is always safest to protect the circuit should the wire break or come in contact with an earth.

Testing!Now you have you display wired up, start the engine!, depending on your cars lambda sensor it may need to be warmed up before it works (even some with heaters need a little help) take the car for a drive & after a few minutes the display should start to react- floor the car & the displays LED’s should go up to green (rich), let-off & they should go to red (lean). Let the car idle, the red / orange LED’s should come on & they will flicker from one side of the display to the other very fast- Most lambda sensors will cycle from rich to lean in about 75 milliseconds and from lean to rich in 100 milliseconds. If the O2 sensor is taking significantly longer to reverse readings, this is a good indication that it is getting sluggish and may need to be replaced

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